Data format error using all-caps, why?

Why would email be returned with the following error, "data format error. Command output:  THEN THE USERNAME IN ALL-CAPS".  

I understand it was returned because it was in all-caps, because in lower case it went through, but why?  And why would it sometimes work?
lloydr1lAsked:
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Irwin SantosComputer Integration SpecialistCommented:
Can you provide more detail?

What Operating System are you using?
What email client? (Outlook/Outlook Express, Eudora?)

By convention, use all LOWERCASE when sending out email.
lloydr1lAuthor Commented:
You use UPPERCASE to emphasize lowercase, cute. ;)

XP SP2, Outlook 2003.

Diagnostic-code: x-postfix; data format error.  Command output:  USERNAME: Mailbox does not exist.  

Oh, and I did leave this out, the user did send to the correct email address, but the automated error message responded with the original email address altered.   Instead of saying user@abc.com could not be found, it stated user@def could not be found.  Two completely different addresses.
Irwin SantosComputer Integration SpecialistCommented:
"You use UPPERCASE to emphasize lowercase, cute. ;)"

Just in case you missed it :)

Is it a specific email account OR many others?
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war1Commented:
Greetings, lloydr1l !

The recipient's server may have a spam rule against username with all caps.  So the server would bounce it.  Check with the recipient's Internet Service Provider.

Best wishes!
lloydr1lAuthor Commented:
Today, with one user, is the first I have become aware of it.  But another user mentioned they had this happen before.
lloydr1lAuthor Commented:
war1, thanks for responding.  The email was sent from our office to corporate and back to someone in our building.
war1Commented:
Is Outlook 2003 Spam filter turned on at recipient?  Lower the spam protection by one level.

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moorhouselondonCommented:
Mailbox names (the bit before the @ sign) are allowed to be case sensitive, therefore it is best to send out using lower case, just to be sure.

Different email handlers treat things in different ways, because the people that designed them do not always look at the de-facto standards.

"The local-part of a mailbox MUST BE treated as case sensitive. Therefore, SMTP implementations MUST take care to preserve the case of mailbox local-parts. Mailbox domains are not case sensitive. In particular, for some hosts the user "smith" is different from the user "Smith". However, exploiting the case sensitivity of mailbox local-parts impedes interoperability and is discouraged."

war1Commented:
lloydr1, any update?
lloydr1lAuthor Commented:
war1, sorry for the delay in responding.  I've been swamped lately, and this problem was very low level.  I think both you and moorhouselondon have offered good reasons why this would have been a problem, unfortunately I don't have time right now to follow up on the problem since email is actually going through.

I just wanted to have an idea what the problem was for future reference, so when I can I will check your suggestion.  In the meantime, points to you both for the good info.

Thanks!
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